The effects of compost and vegetation on stormwater treatment and soil nutrient distribution within bioretention cells
Our research attempts to isolate the effects of low-phosphorus compost and vegetation (Switchgrass)on the performance of bioretention cells in an agricultural landscape. In June 2016, a stormwater bioretention system was installed at the UVM Miller Research Complex, a dairy teaching and research facility comprising of 2.7 acres impervious area, and with uses associated with a working agricultural landscape, including movement of silage, manure, etc. Runoff from the surrounding landscape is captured in grass-lined swales and moves through a settling forebay before it is split among three bioretention cells, each with a unique soil or vegetation treatment. One cell has neither vegetation nor compost, one cell is planted with vegetation (Panicum virgatum) and no compost, and one cell is planted with vegetation and includes a layer of low-phosphorus compost. Low phosphorus compost is defined as being entirely composed of leaves and plant material, and is less than 0.2% total phosphorus by dry weight. The cells are experimentally paired, allowing for a factorial comparison of the compost and vegetation’s affect on performance. Preliminary results have suggested that, after one growing season, the presence of compost and vegetation has no statistically significant effect on nutrient and sediment concentration reduction. The presence of compost does, however, significantly affect above-ground biomass of planted species and the amount of labile phosphorus in the shallow horizon of the of the bioretention soil media. Thus, the use of low-phosphorus compost in bioretention cells may improve plant growth and establishment without contributing significant excess nutrients to effluent, but may load additional phosphorus to the media. We suggest continued monitoring of these cells, as an affect of vegetation may become apparent as the plants continue to grow and the additional phosphorus contributed by the compost to the soil media may decrease the ability for long term sorption and treatment.
Analysis has focused on three components of the Miller Research Center bioretention cells: vegetation, soil media, and water quality.
Vegetation: Plant survivorship, above-ground biomass, and area cover were measured in the two planted bioretention cells in November. A total plant count found that compost had no significant effect on the total number of plants that survive after the first growing season. Harvesting a representative samples of the plants did, however, find that the presence of compost has a significant positive effect on total biomass and area cover. Analysis of the total phosphorus concentration within the harvested above-ground biomass is currently underway.
Soil Media: The concentration of water extractable phosphorus within the soil media of the bioretention cells was measured at the beginning (June) and end (November) of the sampling season. Results have shown that the presence of compost has a statistically significant effect on the concentration of phosphorus in the shallow horizons of the media, which persisted over the time range studied. Future tests on the phosphorus sorption potential of this media is planned to determine if this additional contribution of soluble reactive phosphorus by the compost will have an impact on long term treatment potential.
Water Quality: Concentrations of nitrogen, phosphorus, and total suspended solids were measured at the cells’ influent and separate outflow points to detect significant differences in removal performance with respect to compost and vegetation treatments. Results have shown no statistically significant difference in concentration reduction among the treatments.
The total plant survivorship, above-ground biomass, and percent area cover of the cells with vegetation was performed in November. While this part of the analysis was not originally planned, the difference between treatments was visually apparent and results have thus far been significant. This has been an important part of the research for its consideration of bioretention performance outside water quality and treatment potential. A comparison of the total phosphorus concentration in the aboveground biomass is currently underway, which will allow us to explore plant phosphorus uptake and vitality.
Samples of bioretention media were taken at the beginning and end of the sampling season. A phosphorus sorption index was done for the initial sampling and analysis of the water extractable phosphorus was done for both of these times. Overall, we have found that compost contributes significantly more labile, water-extractable phosphorus to the shallow horizons of the bioretention media and the effect persists over time. A measure of the phosphorus sorption maxima is planned for these cells using the initial media samples, which, coupled with our test of the phosphorus sorption index, will allow us to more thoroughly consider long term phosphorus treatment potential.
Nutrient and sediment concentrations of influent and effluent stormwater were analyzed for thirteen storms between June and November. Thus far, we have found no significant difference in concentration reduction amongst treatments, suggesting that, at this stage, compost and vegetation have no effect on performance. Overall, phosphorus and total suspended solid concentrations of the influent are significantly reduced by greater than 90%. Over the course of the season, we determined that our initial assumption of equal influent volume among the cells was incorrect and that effluent volume is highly impacted by the surrounding native soil. We, therefore, plan to consider the mass loading and removal of the entire system.
Impacts and Contributions/Outcomes
Overall, our study has shown that bioretention is an effective practice for treating nutrients and sediment from agricultural stormwater runoff, regardless of the presence of compost or vegetation. In all three treatments there is a high and statistically significant reduction of phosphorus and sediment concentrations, and a moderate, though not statistically significant, reduction of nitrogen concentration. The presence of compost does, however, have a significant effect on labile phosphorus contribution to the shallow horizons of the bioretention media and the aboveground biomass growth of vegetation. Therefore, while bioretention has an overall positive effect on stormwater quality in an agricultural landscape, the use of compost may have implications for initial plant establishment and long-term phosphorus treatment potential.
This coming year, we plan on publishing our results in a peer reviewed journal and providing outreach to farmers on the possibility of bioretention as a best management practice for treating agricultural runoff. Depending on individual goals, we may suggest any of the three treatments studied. We expect our results to provide flexibility and choice to farmers in treating stormwater runoff from their land as Required Agricultural Practices are implemented in small and medium farming operations across the State of Vermont.
University of Vermont – Plant and Soil Science Department
63 Carrigan Drive
Burington, Vermont 05405
Office Phone: (802) 656-9501
University of Vermont – Rubenstein School of Environment and Natural Resources
81 Carrigan Drive
Burington, Vermont 05405
Farming & Climate Change Coordinator
University of Vermont Extension
375 S Winooski Ave #3
Burlington, VT 05405
Office Phone: 802-656-3495